The present invention relates to ink jet printers and, more particularly, to such a printer in which the jet drop deflection electrode structure is heated above the ambient temperature to preclude drop condensation and accumulation thereon.
As shown in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis, printers of the type to which the present invention relates may generally include a print head which has a fluid receiving reservoir and an orifice plate communicating with the reservoir. The orifice plate defines at least one row, and in a printer such as shown in Mathis, two rows of jet orifices through which ink is forced under pressure to form a plurality of jet drop streams. A fluid ink filament emerges from each of the orifices and drops of substantially uniform size are formed periodically from the filament tips. The regularity of drop formation is enhanced through one of a number of drop stimulation techniques, which may include mechanical stimulation of the entire print head structure or of the orifice plate, as shown in U.S. Pat. No. 3,739,393, issued June 12, 1973, to Lyon et al.
Charging electrodes are positioned adjacent the points at which the drops are formed and induce selectively charged drops in each of the jet drop streams. The jet drop streams thereafter pass through a deflection field which separates the charged and uncharged drops, a portion of the drops being directed in a catch trajectory and the remainder of drops being directed in a print trajectory. In the Mathis printer, two such rows of jet drop streams are provided with a pair of drop catchers positioned parallel to the jet drop stream rows and spaced outwardly therefrom. A deflection electrode is positioned between the rows of jet drop streams and has an electrical deflection potential impressed thereon which deflects the charged drops outwardly such that they strike the catchers. The uncharged drops, however, pass unaffected through the deflection field and strike a print receiving medium, collectively forming a print image thereon. Formation of the print image, therefore, is controlled by controlling charging of the drops in the drop streams.
In order to produce the desired deflection of the charged drops outwardly to the catchers, a relatively high d.c. electrical potential, on the order of 1,000 volts, is applied to the deflection electrode. The catchers are generally formed of electrically conductive material and are typically grounded such that a deflection field is created which extends between each of the catchers and the deflection electrode.
The drop stimulation technique known in the prior art generally result in jet drop streams of uniformly sized drops which are regularly spaced along the stream trajectory. Nevertheless, much smaller drops, termed "satellite drops," may occasionally be formed, giving rise to an ink mist consisting of extremely small drops which may be uncharged or may be charged to varying charge levels of either charge polarity. While very little mist may be present at any one time, the droplets forming the mist may gradually build over a period of time on various portions of the printer structure, including the deflection electrode. Since the ink used in printers of this type is generally conductive, buildup of such ink on the high voltage deflection electrode is extremely undesirable in that the deflection electrode may be shorted to adjacent grounded printer structure by the electrically conductive ink. Shorting of the deflection electrode to associated printer structure for substantial periods may result in excessive current flow from the deflection electrode power supply circuitry, thus overloading the power supply circuit. Additionally, such extended shorting of the deflection electrode may result in the deflection field collapsing such that charged drops are not deflected outward sufficiently to be caught.
Additionally, if a substantial quantity of ink should accumulate at a point on the deflection electrode, a phenomenon known as electrodynamic spraying may occur in which the ink on the deflection electrode is sprayed outward from the electrode in a direction parallel to the deflection field. As will be understood, this may interfere substantially with operation of the ink jet printer and result in deterioration of the print image formed on the print receiving medium. It is seen, therefore, that it is highly desirable to provide an arrangement which prevents buildup of a conductive ink mist on the surfaces of the printer elements.
Larger quantities of ink may also accumulate on the deflection electrode as a result of one or more crooked jet drop streams. This phenomenon occurs most commonly at the beginning of operation of the printer. Additionally, the jet drop streams during printer start up may include drops of a substantially larger size than those that are produced during normal printer operation. Some of these large drops may also strike the deflection electrode. Thus a substantially greater quantity of ink may be deposited on the deflection electrode at the initiation of printer operation than occurs as a result of satellite drop formation during the course of normal printer operation.
U.S. Pat. No. 4,023,183, issued May 10, 1977, to Takano et al, discloses an ink jet printer in which a plurality of cylindrical deflection electrodes are positioned on opposite sides of a single jet drop stream. An electrical potential differential is placed across the cylindrical electrodes such that drops which are charged are deflected in a print trajectory, while uncharged drops pass unaffected through the deflection field and are caught by a catcher arrangement. Each of the deflection electrodes is rotatably mounted with electrode cleaners installed adjacent the electrodes on the opposite sides of the electrodes from the jet drop stream. The electrodes are rotated past the cleaners with the result that any ink mist which may condense upon the electrode surface is removed.
Various drop catcher arrangements have been devised for eliminating buildup of ink mist deposited upon the drop catcher surfaces. As shown in U.S. Pat. No. 3,777,307, issued Dec. 4, 1973, to Duffield, a portion of the catcher may be formed of a porous material with a compartment within the catcher connected to a vacuum source. Ink mist accumulating on the porous surface is therefore drawn into the compartment within the catcher and thereafter removed.
Similarly, as shown in U.S. Pat. No. 4,031,563, issued June 21, 1977, to Paranjpe et al, a relatively thin deflection electrode, arranged to extend between a pair of parallel rows of jet drop streams, has been devised in which the electrode is formed of a porous material which defines a cavity therein. Suction is applied to the ends of the deflection electrode such that any ink mist forming upon the electrode is injected into the electrode cavity and thereafter removed. Although a porous deflection electrode as shown in the Paranjpe et al patent provides substantial advantages over non-porous electrodes, such as shown in the Mathis patent, such porous deflection electrodes may be limited in length, since the electrode must have a substantial partial vacuum maintained along the entire length of the electrode cavity in order to ensure effective drop ingestion. Additionally, it will be appreciated that the minimum thickness of such an electrode is limited by the minimum width of the cavity within the electrode required for sufficient vacuum. Thus, such an electrode necessitates spacing apart the rows of jet drop streams by a certain minimum distance.
U.S. Pat. No. 4,050,377, issued Sept. 27, 1977, to Watanabe et al, discloses a mist printer having an aperture board defining openings therein through which ions pass into an ink mist to charge electrically ink droplets within the mist. The charged ink droplets are attracted to a print receiving medium in a desired pattern by means of an electrode positioned behind the medium. A resistive heating element is positioned within a cavity defined by the aperture board such that the relative humidity within the cavity is reduced. Air is continuously supplied to the cavity within the board and emerges through the openings in the board. As a result of the lowered relative humidity, moisture in the air is prevented from condensing on the board and the exposed metallic portions of the board, therefore, do not accumulate rust.
Thus, it is seen that there is a need for a simple, effective means of reducing condensation on a deflection electrode of the type used in an ink jet printer.